1. Field of the Invention
The present invention relates to the field of optics and, more particularly, to optical couplers for bidirectionally coupling optical radiation between a fiber optic waveguide and a thin-film waveguide.
2. Prior Art
Prior art devices for coupling optical radiation between an optical fiber and a thin-film waveguide are shown in the following patents: U.S. Pat. No. 3,791,715 issued Feb. 12, 1974 to E. G. Lean et al. and assigned to the instant assignee; U.S. Pat. No. 3,774,987 issued Nov. 27, 1973 to L. P. Boivan and assigned to Bell Canada -Northern Electric Research Limited; and U.S. Pat. No. 3,864,019 issued Feb. 4, 1975 to G. Smolinsky et al and assigned to Bell Telephone Laboratories, Inc.
In the Lean et al patent, light in a fiber optic is coupled into a thin-film waveguide through use of resonant cavity and a selectively producible grating between the cavity and the thin-film waveguide. The coupling is unidirectional, viz., from fiber to waveguide and employs the resonant cavity.
In the Boivan et al patent, the optical fiber is axially aligned with the thin-film waveguide by mounting it in a crystalographically etched V-groove in the substrate, upon which is deposited a thin-film waveguide. Coupling is from the waveguide to the fiber through air.
The patent to Smolinsky et al provides bidirectional coupling between a thinfilm waveguide and an optical fiber by inserting the fiber into a drilled hole in the substrate with a filler material added to form a hemispherical lens. The light in the substrate, whether it comes from the film waveguide or the optical fiber, is essentially unguided and subject to critical geometric relativities, as well as being difficult to fabricate.
The instant invention, unlike Smolinsky et al, provides complete guided mode structures in the waveguide, the optical fiber, and in the coupling region per se. Smolinsky et al. couple with unguided light in the substrate with a potential prejudice to the coupling efficiency. High coupling efficiency is possible only with angle or low mode number guides. The leaky wave coupling and the complete guided mode coupling of the instant invention provide high efficiencies even with multi-mode guides.